Surface emitting laser diodes (hereinafter referred to as surface-emitting LDs) are classified into three types by the shapes of their cavity resonators: vertical cavity surface-emitting LD, horizontal cavity surface-emitting LD, and bending cavity surface-emitting LD, as disclosed in the Journal of Electronic Information Communication Institute, C-I, Volume J75-C-I, Number 5, pages 245-256 (May 1992). These surface-emitting LDs emit laser light in a direction perpendicular to a main surface of the substrate whereas conventional LDs emit laser light parallel to a main surface of the substrate.
Among those three types of surface-emitting laser diodes, the horizontal cavity surface-emitting LD is produced in a relatively simple process including fabricating a diffraction grating in an optical waveguide, a technique usually employed in production of a DFB (Distributed Feedback) laser diode or a DBR (Distributed Bragg Reflector) laser diode. The horizontal cavity LD is produced so that the diffraction grating is a second order grating, i.e., having a period equal to .lambda..sub.0 (wavelength of laser light)/n(effective refractive index of waveguide). In the following description, this second-order diffraction grating is called a secondary diffraction grating.
FIG. 15 is a perspective view illustrating a prior art surface-emitting DBR laser disclosed in Applied physics Letters, 50(24), 15 Jun. 1987, pages 1705-1707. In the figure, a surface-emitting DBR laser 150 includes an n type GaAs substrate 52. There are successively disposed on the n type GaAs substrate 52 an n type GaAs buffer layer 53, an n type AlGaAs cladding layer 54, and a multiquantum well (hereinafter referred to as MQW) lightguide layer 55. The MQW lightguide layer 55 comprises alternating GaAs well layers and AlGaAs barrier layers. A p type AlGaAs cladding layer 56 having a stripe-shaped secondary diffraction grating 58 is disposed on the lightguide layer 55. A p type GaAs contact layer 59 is disposed on a part of the secondary diffraction grating 58 in a laser oscillation region 150A. Insulating films 57 are disposed on the cladding layer 56 at opposite sides of the stripe-shaped secondary diffraction grating 58, on the top surface of the diffraction grating 58, except for the laser oscillation region 150A, and on the opposite side surfaces of the diffraction grating 58 and the contact layer 59 in the-laser oscillation region 150A. A p side electrode 61 is disposed on the insulating film 57 and the contact layer 59 in the laser oscillation region 150A. A facet of the laser oscillation region 150A is coated with a high reflectivity film 60.
When current is injected into the laser oscillation region 150A through the p side electrode 61, fundamental laser oscillation occurs. When the laser light reaches the p type AlGaAs cladding layer 56 including the secondary diffraction grating 58, i.e., the optical waveguide, maximum reflectivity occurs at a wavelength determined by the period of the secondary diffraction grating 58, and a resonator is produced between the lightguide layer 55 and the high reflectivity coating film 60, whereby the laser oscillates in a single longitudinal mode. The secondary diffraction grating 58 converts laser light propagating parallel to the surface of the substrate 52 into laser light propagating perpendicular to the surface of the substrate and outputs the laser light.
When a plurality of horizontal cavity LDs, such as the above-described surface-emitting DBR LDs 150, are two-dimensionally integrated on the same substrate, a surface-emitting DBR LD array is realized.
FIG. 16 is a perspective view illustrating a prior art surface-emitting DBR LD array disclosed in Electronics Letters, Volume 24, Number 5, 1988, page 283. In the figure, the same reference numerals as in FIG. 15 designate the same or corresponding parts. A two-dimensional surface-emitting DBR LD array 160 includes three surface-emitting DBR LDs respectively including secondary diffracting gratings 58a, 58b, and 58c arranged parallel to each other on the same substrate 52, whereby the output power of the laser light is increased.
In this prior art surface-emitting DBR LD array 160, output laser light from the LD array 160 comprises laser light emitted from the respective laser resonators so that higher output power is obtained as compared to the DBR surface-emitting LD 150 including a single laser resonator (secondary diffraction grating) shown in FIG. 15. However, since a plurality of laser oscillating regions are disposed close to each other, when these lasers oscillate continuously, the temperature of the device significantly increases so that the refractive index in the optical waveguide, i.e., the lightguide layer and the cladding layer, unfavorably varies. This variation in the refractive index causes a difference between the Bragg wavelength of the secondary diffraction grating in the optical waveguide and the laser oscillation wavelength, resulting in an unstable beam output angle.
FIGS. 17 (a) to 17 (c) illustrate a surface-emitting DBR LD array including an annular diffraction grating disclosed in Japanese Published Patent Application Hei. 3-257888, wherein FIG. 17 (a) is a perspective view of the DBR LD array, FIG. 17(b) is a sectional view taken along line 17b-17b of FIG. 17 (a), and FIG. 17 (c) is a sectional view taken along line 17c-17c of FIG. 17 (a).
In these figures, a surface-emitting DBR LD array 170 includes an n type InP substrate 70 having opposite front and rear surfaces. An n type InGaAsP waveguide layer 83 having a band gap energy equivalent to a wavelength of about 1.3 .mu.m, an InGaAsP active layer 82 having an energy band gap equivalent to a wavelength of about 1.55 .mu.m, a p type InP cladding layer 81, and a p type InGaAsP cap layer 80 are successively disposed on the n type InP substrate 70. Portions of these layers 80, 81, and 82 in the center of the structure are selectively removed, and an annular secondary diffraction grating 79 is produced at the exposed surface of the n type InGaAsP waveguide layer 83. Further, these layers 80, 81, 82, and 83 are arranged in a plurality of stripe-shaped mesas across the diffraction grating 79. More specifically, as shown in FIG. 17 (a), four stripe-shaped laser resonators 71, 72, 73, and 74, each having the cross-section shown in FIG. 17 (b), are produced by selectively removing portions of the layers 80 to 83. As shown in FIG. 17 (c), a semi-insulating InP layer 84 is disposed on the substrate 70, contacting opposite sides of each laser resonator. An n side electrode 78 is disposed on the rear surface of the substrate 70. A p side electrode 75 is disposed on the stripe-shaped resonators 71 to 74 and on the semi-insulating InP layer 84. Reference numeral 81a designates facets, numeral 76 designates an insulating film, and numeral 77 designates a metal film.
In this prior art surface-emitting DBR LD array, laser light produced by laser oscillations in the respective resonators 71 to 74 is output upward from the annular diffraction grating 79 so that a high output power is obtained from the single aperture in proportion to the-number of the laser resonators. In addition, the space between adjacent resonators is larger than that of the DBR LD array shown in FIG. 16. Therefore, the unwanted increase in the temperature of the device during the continuous laser oscillation is suppressed, whereby the instability of the beam output angle is reduced to some extent.
Since the prior art surface-emitting DBR-LD array outputs a high power laser light in a direction perpendicular to the main surface of the substrate, it is employed as a semiconductor light emitting element for optical interconnection of signals between a plurality of computers or for optical interconnection of signals in a computer, i.e., signals between a plurality of boards or on each board or signals between a plurality of chips. This optical interconnection system requires means for converting electrical signals into light, means for transmitting optical signals, and means for converting optical signals into electrical signals. A semiconductor light emitting element (for example, a semiconductor laser), an optical waveguide, and a semiconductor light responsive element are respectively employed for those means. FIG. 18 (a) is a schematic diagram illustrating an optical interconnection system using an optical waveguide (hereinafter referred to as optical waveguide interconnection), and FIG. 18 (b) is a schematic diagram illustrating an optical interconnection system using no waveguide (hereinafter referred to as spatial optical interconnection).
As shown in FIGS. 18 (a) and 18 (b), in the optical waveguide interconnection and the spatial optical interconnection, the direction in which laser light emitted from the semiconductor laser is guided is changed by an optical waveguide or by a mirror. Therefore, as shown in FIG. 19, if the directions of laser light beams 92a to 92c output from surface-emitting LD arrays 90a to 90c disposed on a transmitter chip 190a are controlled separately for each surface-emitting LD array, physical means for changing the optical path, such as the waveguide or the mirror, can be eliminated. In addition, information from those three surface-emitting LD arrays 90a to 90c is received by six photodiodes 91a to 91f disposed on a receiver chip 190b so that the device size is significantly reduced. In the prior art surface-emitting LD array shown in FIG. 16, however, a plurality of laser resonators are arranged parallel and close to each other, and each laser resonator is adversely affected by light leakage from the adjacent laser resonator. Therefore, even when the respective laser resonators oscillate with different driving currents and a composite wave is produced, the respective laser resonators do not operate stably so that the output direction of the obtained phase composite wave cannot be arbitrarily controlled.
On the other hand, in the prior art surface-emitting LD array 170 shown in FIG. 17 (a), since a plurality of laser light beams resulting from oscillations in the respective laser resonators 71 to 74 interfere with each other at the annular diffraction grating 79, a phase composite wave is not obtained so that the output direction of the laser light from the respective laser resonators cannot be arbitrarily controlled.
Meanwhile, multiple wavelength optical communication in which multiple wavelength light comprising laser light beams of different wavelengths is guided through an optical fiber has been recently achieved. In the surface-emitting LD array shown in FIG. 16, if the pitches of the respective secondary diffraction gratings 58a to 58c are different from each other, laser light beams with different wavelengths are output at the same time. However, in the prior art surface-emitting LD array shown in FIG. 16, the secondary diffraction gratings 58a to 58c are parallel and close to each other so that laser light beams emitted from the respective diffraction gratings interfere with each other so that multiple wavelength laser light having prescribed wavelengths cannot be stably output. In the prior art surface-emitting LD array shown in FIG. 17 (a), laser light beams with different wavelengths interfere with each other at the annular diffraction grating 79 so that multiple wavelength laser light having prescribed wavelengths cannot be stably output.